The present exemplary embodiment relates to document processing systems. It finds particular application in conjunction with sensing and control of banding and will be described with a particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
In a typical printing system, a photoconductive drum or photoreceptor rotates at an angular velocity. As the photoconductive drum rotates, the photoconductive drum is electrostatically charged. A latent image is exposed line by line onto the photoconductive drum using a scanning laser, e.g., using a rotating polygon mirror. The latent image is developed by electrostatically adhering toner particles to the photoconductive drum. The developed image is transferred from the photoconductive drum to the output media such as paper. The toner image on the paper is fused to the paper to make the image on the paper permanent. The surface of the photoconductive drum is cleaned to remove any residual toner on the surface of the photoconductive drum.
Typically, the printing device drives the photoconductive drum using a motor drive system or a motor train. The motor drive system, which drives the photoconductive drum, has a substantial amount of external loading, because the motor drive system typically drives the auxiliary rollers and transports the paper through a series of gear trains. With the additional external loading, as well as periodic disturbances due to imperfections in the series of gear trains, the motor drive system imparts a varying velocity on the photoconductive drum.
The varying photoconductive drum velocity causes scan line spacing variation in the printed image. The scan line spacing variation is a significant contributor of artifacts in marking process. For example, halftone banding caused by scan line spacing variation is one of the most visible and undesirable artifacts, appearing as light and dark streaks across a printed page perpendicular to the process direction. Banding generally occurs across the full width of an image, and may vary in amplitude in time and in the direction perpendicular to the marking process direction, i.e., the cross-process direction. Often the dominant banding defect source (or sources) are well known ahead of time based on mechanical design of the printing system. For example, the banding can occur due to a motion quality error due to runout of a roll, gear teeth meshing errors, ROS polygon once around errors, and the like.
However, the periodic bands are generally not synchronous with the image. Thus, while each image may have the same banding frequency and amplitude, the banding phase relative to the image differs from one print to another.
One approach to eliminate banding defects is to require the manufacture of parts/subsystems to meet tight tolerances which results in high costs.
Another approach is to measure velocity at various points in a mechanical drive train or at the photoreceptor drum and compensate for the velocity variation.
There is a need for methods and apparatuses that overcome the aforementioned problems and others.